This is a nasty one. Not only do these things hit some of the phobia buttons I’ve discussed before, but they’re smart enough to understand basic engineering (one of the scariest things you can say about any monster is “it can think”), and reasonable enough to work together with other monsters that share a common interest (the scariest thing you can say is “they’re working together”).
Honestly, given all that, the ability to cast Baleful Polymorph just seems gratuitous.
The Auñ Pana are evil man-eating fish from the folklore of the Yanomami of Brazil and Venezuela. They are large, have arms, and are covered with hair. Apparently they also have some degree of magical power. The auñ pana live in deep water and school with Pehiwetinome, which are equally large and anthropophagous.
A group of auñ pana and pehiwetinome once tore down a bridge that the Yanomami were crossing by biting through its wood. The bridge collapsed and became a raft, and the surviving Yanomami were turned into monkeys and pigs.
Albert, B.; Becher, H.; Borgman, D. M.; Cocco, L.; Colchester, M. E. M.; Finkers, J.; Knobloch, F.; Lizot, J.; and Wilbert, J.; Wilbert, J. and Simoneau, K. eds. (1990) Folk Literature of the Yanomami Indians. UCLA Latin American Center Publications, University of California, Los Angeles.